This proposal is a continuum of investigative activities in this laboratory which define the characteristics of the membrane transport of the antifols and their interaction with target site(s) within the intact cell. These studies explore basic and applied aspects of MTX (methotrexate) pharmacokinetics with a current emphasis on developing a better understanding of the basis of the efficacy of high-dose MTX regimens. Properties of the MTX-cell interaction will be correlated with growth characteristics of mammalian cell populations in culture emphasizing the critical role that free intracellular MTX plays in achieving the biochemical and cytotoxic effects of this agent. The nature of the energy-dependent processes which inhibit accumulation of free intracellular MTX will be evaluated. The mechanism by which vincristine augments cellular uptake of MTX and potentiates inhibition of DNA synthesis by MTX will be further characterized. A role for polyglutamates of MTX as factors in MTX cytoxicity will be evaluated as well as the cellular pharmocokinetics of these compounds as well as diesters of MTX. The nature of the interaction between MTX and DHFR (dihydrofolate reductase) within the cell will be explored. Studies will evaluate whether the requirement for free intracellular MTX to inhibit tetrahydrofolate synthesis may be related to a form of DHFR unassociated with NADPH. The cellular pharmacokinetics of aminopterin will be studied to determine whether there is a role for this agent in high-dose regimens with folinic acid rescue. Finally, aspects of the transport of MTX and other folate compounds will be evaluated including the nature of multiple transport routes for these compounds, characteristics of MTX autoexchange, and the thermodynamics of MTX transport with particular regard to the possibility of an exit pump for MTX and other folate compounds. Other studies will further assess the role of cellular microtubules in energy-dependent transport processes with the characterization of the mechanism by which microtubular inhibitors reduce uphill transport of alpha-aminoisobutyric acid in Ehrlich ascites tumor cells.